In recent years, chip type electronic parts used for surface mounting, such as ICs, transistors, diodes, condensers, piezoelectric resistors and the like have been supplied contained in packaging materials consisting of a plastic carrier tape having at given intervals pockets formed by embossing so as to accommodate chip type electronic parts of particular shape and a cover tape heat-sealable to the carrier tape. The electronic parts contained in the packaging materials are automatically taken out after peeling the cover tape of the package, and are mounted on the surface of an electronic circuit substrate.
The strength when the cover tape is peeled off from the carrier tape is called "peel-off strength". When the peel-off strength was too low, there was a problem that the cover tape was separated from the package at the time of the package transfer and the contents, i.e., the electronic parts fell off. When the peel-off strength was too high, there occurred a phenomenon that the carrier tape underwent vibration in peeling the cover tape and, as a result, the electronic parts jumped out of the pockets right before their mounting, i.e., jumping trouble.
As a material for the carrier tape, there have conventionally been used polyvinyl chloride (PVC) sheet, polystyrol sheet, polyester (PET) sheet, polycarbonate sheet, acrylic sheet and other sheets, all of which are readily formed into sheets. Meanwhile, as a cover tape heat-sealable to the carrier tape, there has generally been used a composite film obtained by laminating a polyethylene-modified film or an ethylene-vinyl acetate copolymer (EVA)-modified film which is heat-sealable for a PVC or polystyrol sheet to a biaxially oriented polyester film. In the conventional cover tape, however, the peel-off strength was greatly affected by the sealing conditions such as sealing temperature, sealing pressure and the like, and the fluctuation in sealing conditions made it difficult to control the peel-off strength in an appropriate range as mentioned above; further, the peel-off strength was also affected by temperature and humidity depending upon the storage conditions of the conventional cover tape or the storage conditions after sealing, whereby the peel-off strength was increased or decreased with the lapse of time and, in some cases, deviated from an appropriate range.
There are two types of cover tapes which differ in peeling mechanism; one is an interface peeling type and the other is a cohesion peeling type. In the case of the interface peeling type, it was difficult to satisfy two contradictory properties of "strong adhesion and easy peeling" which are basic requirements for cover tapes, because the sealing surface and the peeling surface are the same; the peel-off strength was affected by the surface condition of a carrier tape to which a cover tape was to be sealed, and consequently fluctuated and thus it was difficult to control the peel-off strength in an appropriate range as mentioned above; the peel-off strength was also affected by temperature and humidity depending upon the storage conditions of a cover tape or the storage conditions after sealing; whereby the peel-off strength was increased or decreased with the lapse of time and, in some cases, deviated from an appropriate range. Meanwhile, in the case of the cohesion peeling type, by using a film as disclosed in Japanese Patent Publication No. 12936/1986, it was possible to produce a cover tape wherein the sealing surface and the peeling surface were different and the peel-off strength was obtained by the cohesive strength of the adhesion layer; this cover tape showed excellent properties which were stable even with the lapse of time. However, the cohesive strength of the adhesion layer was not so large; therefore, when the cover tape as a product was stored in a rolled state particularly at high temperatures in summer, etc., the biaxially oriented film caused blocking with sealing surface at the core portion, which caused insufficient sealing, significant reduction in transparency, etc. Further, regarding "strong adhesion and easy peeling", easy peeling could be attained by cohesion peeling, but strong adhesion still had a problem because the adhesion layer did not strongly adhere to a carrier tape and the adhesion was greatly affected by the sealing surface condition and sealing width of the carrier tape.
With the significant improvement in surface mounting technique in recent years, electronic parts to be surface-mounted have come to be made in chips of higher capability and smaller size. In such a movement, the electronic parts, when transferred in a package, have undergone vibration and contact with the embossed inner surface of the carrier tape of the package or the inner surface of the cover tape of the package, and the resulting friction has generated static electricity; static electricity has been also generated when the cover tape was peeled off from the carrier tape; this static electricity has generated sparks to cause destruction and deterioration of the electronic parts. Thus, the electronic parts have had troubles due to static electricity, and it has been the most important task to develop an antistatic measure for packaging materials such as carrier tape and cover tape.
The antistatic treatment for a carrier tape has hitherto been effected by incorporation or coating of carbon black into or on the carrier tape material used, and the effect has been satisfactory. However, no sufficient measure has yet been taken for the antistatic treatment for a cover tape, and currently there is merely effected, for example, coating of an antistatic agent or a conductive material on the outer layer of the cover tape. This treatment, however, has not been sufficient for the protection of electronic parts to be contained by sealing, because the treatment is applied to the outer side of the cover tape, and the treatment was ineffective particularly for the static electricity generated by the contact of the inner surface of the cover tape with the electronic parts. The antistatic treatment for the inner surface of the cover tape, i.e. the adhesion layer of the cover tape can be effected by coating or incorporation of an antistatic agent on or into the adhesion layer. In the case of incorporation, however, the antistatic agent incorporated into the adhesion layer has bled onto the inner surface of the cover tape and invited unstable sealing and many troubles due to poor sealing; moreover, the antistatic effect has decreased with the lapse of time, has been greatly affected by the conditions under which the package was used, i.e., temperature and humidity, particularly humidity, and has significantly decreased under a low humidity such as 10% R.H.; thus, no sufficient effect has been obtained. Meanwhile, the incorporation of a conductive material into an adhesion layer has been technically difficult, because the adhesion layer has been formed by laminating an extruded film or the like to an outer layer; moreover, the incorporation has significantly reduced the transparency of the resulting cover tape, making the cover tape usability questionable. The coating of a conductive material on an adhesion layer has not been effected in fact, because the selection of a binder stably bondable to the carrier tape has been difficult, and because the adhesion layer is covered and hidden by the coating.